mt76: connac: always check return value from mt76_connac_mcu_alloc_wtbl_req

Even if this is not a real bug since mt76_connac_mcu_alloc_wtbl_req routine
can fails just if nskb is NULL , always check return value from
mt76_connac_mcu_alloc_wtbl_req in order to avoid possible future
mistake.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>